<quote>
The VRs that are not a string of
characters and consist of multiple bytes are:
 2-byte US, SS, OW and each component of AT
 4-byte OF, UL, SL, and FL
 8 byte FD
Note: For the above VRs, the multiple bytes are presented in
increasing order of significance when in Little
Endian format. For example, an 8-byte Data Element with VR of FD,
might be written in hexadecimal as
68AF4B2CH, but encoded in Little Endian would be 2C4BAF68H.
</quote>

When a defined length Sequence (public or private) is sent as UN in a
BigEndianExplicit DataSet, it shall be byte swapped, since the
received dataset was Implicit VR little endian in its previous form
(otehrwise UN would not have been needed). Following the defined term
in cp 246, this SQ is an IVRLE.

I did check:
- dcmtk 3.5.4 / gdcm 2.0.9 is not doing it
- dicom3tools (latest) is doing it